Allow "using" to coexist with other vpart reqs

There were two ways to specify vehicle part installation requirements.
Either with "using" or with standard requirements ("components",
"qualities", and "tools").  Previously the two were mutually exclusive,
but some parts tried to use both.  There's no reason we should prevent
that, so allow both to be used.

src/veh_type.cpp

@@ -192,11 +192,13 @@ static void parse_vp_reqs( JsonObject &obj, const std::string &id, const std::st
             reqs.emplace_back( requirement_id( cur.get_string( 0 ) ), cur.get_int( 1 ) );
         }
 
-    } else {
-        const requirement_id req_id( string_format( "inline_%s_%s", key.c_str(), id.c_str() ) );
-        requirement_data::load_requirement( src, req_id );
-        reqs = { { req_id, 1 } };
     }
+
+    // Construct a requirement to capture "components", "qualities", and
+    // "tools" that might be listed.
+    const requirement_id req_id( string_format( "inline_%s_%s", key.c_str(), id.c_str() ) );
+    requirement_data::load_requirement( src, req_id );
+    reqs.emplace_back( req_id, 1 );
 }
 
 /**